1. Field of the Invention
The invention relates to self expanding stents and stent-grafts. More particularly, the invention relates to stents and stent-grafts with the stent having a polymeric coating which provides enhanced hoop strength as well as other benefits.
2. State of the Art
Transluminal prostheses are well known in the medical arts for implantation in blood vessels, biliary ducts, or other similar organs of the living body. These prostheses are commonly known as stents and are used to maintain, open, or dilate tubular structures or to support tubular structures that are being anastomosed. When biocompatible materials are used as a covering or lining for the stent, the prosthesis is called a stent-graft. If used specifically in blood vessels, the stent-graft is known as an endovascular graft. A stent or stent-graft may be introduced into the body by stretching it longitudinally or compressing it radially, until its diameter is reduced sufficiently so that it can be fed into a catheter. The stent-graft is delivered through the catheter to the site of deployment and then released from the catheter, whereupon it self-expands. Stent-grafts introduced in this manner are known as endoluminal stent-grafts.
A typical state of the art stent, such as disclosed in U.S. Pat. No. 4,655,771 to Wallsten or in U.K. Patent Number 1,205,743 to Didcott, is shown herein in prior art FIGS. 1, 1a, 2, and 2a. Didcott and Wallsten disclose a tubular body stent 10 composed of wire elements, e.g. 12, 13, each of which extends in a helical configuration with the centerline 14 of the stent 10 as a common axis. Half of the elements, e.g. 12, are wound in one direction while the other half, e.g. 13, are wound in an opposite direction. With this configuration, the diameter of the stent is changeable by axial movement of the ends 9, 11 of the stent. Typically, the crossing elements form a braid-like configuration and are arranged so that the diameter of the stent 10 is normally expanded as shown in FIGS. 1 and 1a. The diameter may be contracted by pulling the ends 9, 11 of the stent 10 away from each other as shown by the arrows 16, 18 in FIG. 2. When the ends of the body are released, the diameter of the stent 10 self-expands and draws the ends 9, 11 of the stent closer to each other. The contraction to stretching ratio and radial pressure of stents can usually be determined from basic braid equations. A thorough technical discussion of braid equations and the mechanical properties of stents is found in Jedweb, M. R. and Clerc, C. O., "A Study of the Geometrical and Mechanical Properties of a Self-Expanding Metallic Stent--Theory and Experiment", Journal of Applied Biomaterials; Vol. 4, pp. 77-85 (1993). In general, however, the contraction to stretching ratio is related to the axially directed angle .alpha. between the crossing elements 12, 13 in the expanded state as shown in FIG. 1. As explained in Didcott, the greater the magnitude of the angle .alpha., the greater the amount of axial extension will be required to contract the diameter of the stent.
The ability of a stent to withstand radial forces is known in the art as "hoop strength". The hoop strength of both the Wallsten and the Didcott stents is relatively low. The Wallsten stent provides an improvement in hoop strength over Didcott by virtue of the higher pitch angle (.alpha.&gt;90.degree.). However, the higher pitch angle of the Wallsten stent renders the stent more difficult to place since substantial elongation is required to pull the stent down into a catheter introducer. Various designs have been advanced in efforts to increase stent hoop strength. These designs include the use of thicker wires, the use of more wires, and the use of paired wires. However, there are limitations to each of these designs. For example, if too many wires are used or if the wire diameter is too large, the stent will tend to demonstrate a taper on one end and a flare on the other end. This is detrimental to stent performance. Moreover, the use of numerous and/or thick wires often results in wire jamming when the stent is drawn down. This requires a larger introducer catheter which renders it more difficult to place in distal and tortuous vessels.
Apart from hoop strength, another problem with conventional stents is that the ends fray or become unbraided when they are cut. When this happens, it becomes difficult to load the stent into an introducer and it is possible for a stray wire end to penetrate the wall of the introducer. Similarly, an unbraided wire end can perforate the human vessel during or after placement.
Still another problem with the state of the art stents is that during normal use, even without cutting the stent, the ends of the stent tend to taper inward due to slippage of the wires and loss of braid structure. The tapered ends of a stent can perturb flow through the lumen of the stent and cause thrombosis. In addition, as the ends of an installed stent taper inward, the stent can become dislodged and may even be washed downstream through the vessel in which it was installed.
Yet, another problem with conventional Didcott or Wallsten stents is illustrated in prior art FIG. 3. When a stent 10 of this type is deployed in a vessel 20 having a bend 22, the pitch angle of the wired is increased in the portion of the stent 10 which traverses the bend 22. Hence, the diameter of the stent 10 at the center of the bend 22 is larger than the diameter of the stent 10 at its ends 9, 11, as the center of the stent stretches the vessel at the bend. This tends to alter the hemodynamics of the vessel.
Still another problem associated with these aforementioned stents is that the stent will flex continuously with each bolus of blood passing through the stent. The flexion continues until the stent is totally ingrown with biological tissue. During flexion, the wires undergo a scissors-like activity at the crossover points which can irritate tissue and adversely affect patency, especially in small diameter vessels such as the coronary arteries. Moreover, the points where the wires cross over each other are subject to abrasion when the stent is flexed in the vasculature. Severe abrasion manifests as wear in the wires which can ultimately lead to premature breakage of the wire components.
Another kind of (non-braided) stent is disclosed in European Patent Publication No. 0312852 to Wiktor. A Wiktor-type stent 30 is shown in prior art FIG. 4 in conjunction with a balloon catheter 31. The stent 30 is made of a single strand of zig-zag filament 32 which is helically wrapped around a mandril. While the filament 32 does not necessarily cross over itself, adjacent zig-zags, e.g. 34, 36, touch each other or come close to touching each other. One of the disadvantages of the Wiktor-type stent is that the zig-zag wire tends to expand non-uniformly when expanded in an artery by a balloon catheter. In addition, the non-braided stent can unfurl during maneuvering the balloon catheter in the vasculature which can cause placement problems as well as damage to the endothelium. In addition, the hoop strength of the Wiktor-type stent is relatively low.
Further disadvantages of conventional wire stents are that they are intrinsically thrombogenic and do not bind well to surface coatings due to the inertness of the metallic oxide layers on the wires.